During the last few years, a considerable effort has been made to almost completely replace tube mills and ball mills with other grinding processes. One of these processes for crushing or grinding brittle material is the procedure illustrated in FIGS. 4 and 6 of European Patent document EP-B-O 084 383, wherein the material to be ground is pressed under high pressure between the roll gap of a high pressure roller press, which, on the one hand, causes a disintegration of particles and, on the other hand, produces a cracking on the inside of the particle, leading to a visible formation of agglomerates (so-called scabs). The separate particles of the bed of material to be ground are crushed in the area of the smallest roll gap of the high-pressure roller press. In other words, the load, or bed of material is crushed as it squeezed together in passing between the rollers. This process is called material crushing or autogenous size reduction. The material agglomerates produced by the high-pressure roller press are disagglomerated or broken up and deposited onto a sifter whose grit is completely recirculated to the high-pressure roller press. Because of the high amount of pressure applied inside the high-pressure roller press and the destruction of the grain structure, the material agglomerates coming out of the roller press contain fine finished material which together with the sifter air will be extracted by the sifter of the circulating grinding plant.
For the previously known recirculating grinding plant, it has been suggested not to disagglomerate the scabs deposited by the high-pressure roller press in a subsequently arranged tube mill, as it is commonly done, but to disagglomerate the scabs with a sizing screen prior to depositing the scab material onto the sifter. If the procedure calls for grinding cement clinker into cement, for example, then the throughput capacities of the sizing screens as demanded by the cement industry, can no longer be handled. Furthermore, the screen material of these sizing screens which is normally used for this purpose would quickly wear out, because the material of the scabs that were pressed by the high-pressure roller press consist of granules that have a comparably sharp-edged grain surface which would even further promote the wear and tear of a sizing screen. This wear problem which accelerates with increasing pressure applied by the roller press, also applies to the sifter, especially if it is a dynamic sifter with at least one rotating rod basket, whose rod-shaped turbo-elements are subject to an even higher wear and tear due to the impact strain, in particular because of the coarse and sharp-edged granules.
Under actual operating conditions, when using a material crushing high-pressure roller press for energy saving purposes, the raw material (if necessary, together with the sifter grit) is deposited onto the roller press, which was operated using a very high pressure. The wear problems for the roller press, the disagglomerator, and the sifter are tremendous when using this process. On the other hand, when grinding and drying (grind-drying) wet and brittle material, such as raw material to be used for cement raw meal by utilizing a high-pressure roller press (such as disclosed in European patent document EP-B-O 220 681), the fresh and wet material will not be processed by the roller press, but first by an impact pulverizer where the material will be crushed, pre-dried, and the material scabs coming from the high-pressure roller press are being disagglomerated at the same time, whereby only the grit of the sifter coming from the recirculating grinding plant will be transported to the delivery chute of the roller press. In other words, this known rotating grinding plant is operating with two mills, which is relatively expensive as far as investment and operating costs are concerned, and its design is more suitable for processing wet raw material. Even for this rotating grinding plant efforts are being made to achieve a possibly high pressing effect on the material in the gap of the roller press, which requires the use of a disagglomerator for disagglomerating the press scabs, and that by itself increases investment and operating costs.